1. Field of the Invention
This invention relates to an apparatus for controlling mass flow of various gases and the like with high precision, and more specifically to a mass flow controller having improvements in a passageway thereof through which gases and the like flow and in assemblage of various parts disposed in such passageway.
2. Description of Related Art
Recently various gases are used in manufacturing processes of, for example, semiconductors, and in this connection the demand for more precisely measuring and controlling flow of the gas used becomes greater.
A mass flow sensor which has been, in general, employed heretofore has such construction, as shown in FIG. 3, that a capillary tube 3 having a diameter of 0.25-0.75 mm is externally provided on a conduit 1 for fluid in order to communicate opposite ends of a by-passed portion 2 positioned inside the fluid conduit 1, and an outer circumferential portion of the capillary tube 3 is wound with a self exothermic resistor 4 made of platinum or the like and which is connected with a bridge circuit. The self exothermic resistor 4 is a temperature sensor which detects a temperature difference between the upstream side and the downstream side of the capillary tube 3 in case of heat generation. When a prescribed amount of a gas is introduced into the capillary tube 3 from the fluid conduit 1, a quantity of heat which is proportional to the mass flow of the gas transfers from the upstream side to the downstream side, whereby values of resistance at a site 4a on the upstream side and a site 4b on the downstream side of the self exothermic resistor 4 differ from one another, and such variation is electrically detected by means of the bridge circuit.
In such mass flow sensor, however, since the self exothermic resistor 4 wound around the outside of the capillary tube 3 as a temperature sensor, the speed of response for detecting change in flow is slow, and it sometimes happens that the speed of response in a flow regulating valve is faster than the former speed of response, so that there occurs so-called overshooting and the like. Furthermore, there is a problem that, since the exothermic resistor 4 usually is maintained at a high temperature of 150.degree. C.-200.degree. C. in order to increase the speed of response in such sensor as described above, a gas flowing through the capillary tube 3 is decomposed thermally during the course of flow, dependent upon the type of such gas, so that a decomposition product adheres to an inner wall or the like of the capillary tube 3, whereby clogging and the like occurs.
There has been developed a mass flow sensor the response of which has been significantly improved with respect to changes in temperature due to a construction that a gas passageway is defined on a silicon chip by utilizing a photolithographic technique, which is a manufacturing technique for semiconductors, a thin film-like temperature sensor is formed in the gas passageway, and the resulting silicon tip is directly disposed in a conduit for fluid, and such mass flow sensor has been put to practical use.
An example of a conventional mass flow controller wherein such mass flow sensor as described above is used is shown in FIG. 4 in which a base block 6 having a filter 5 is mounted on and connected to the midportion of a fluid conduit 1, and a mass flow control unit 7 is vertically disposed at a rearward position of the filter 5 in the base block 6. The mass flow control unit 7 is constructed such that a cylindrical housing 8 is provided with a by-pass 2, a mass flow sensor 10 and a flow regulating valve 11. An inlet port 8b and an outlet port 8c are defined in a reduced diameter portion 8a in the lower portion of the housing 8. These parts are arranged such that the inlet port 8b and the outlet port 8c are communicated with a flow through passage 6b in the base block 6 when the reduced diameter portion 8a of the housing 8 is fitted into a fitting hole 6a provided at the rear of the filter 5 in the base block 6 through an O-ring 12. A valve casing 11a for flow control valve 11 is internally provided at an area extending from an expanded diameter portion 8d of the housing 8 to the reduced diameter portion 8a positioned therebelow. A forward end portion of the valve casing 11a is communicated directly with the outlet side of the flow through passage 6b through the outlet port 8c. An area defined between the valve casing 11a and the reduced diameter portion 8a of the housing 8 is utilized as a gas passageway 13, and gas which passes through the passageway 13 from the inlet port 8b flows through the expanded diameter portion 8d via a communicating hole 11b defined on the upper part of the valve casing 11a. A by-pass casing 2a having an upwardly open shape is rigidly provided on the upper part of the valve casing 11a, and an upper lid 14 of the housing 8 is disposed on the upper part of the by-pass casing 2a. A space defined between the outer circumference of the by-pass casing 2a, the housing 8, the upper part of the by-pass casing 2a and the upper lid 14 is utilized as a gas passageway 15, and gas which passes through the passageway 13 flows into the by-pass casing 2a via the passageway 15. A plurality of by-pass elements are provided together with mass flow sensor 10 in the by-pass casing 2a, and the mass flow sensor 10 is electrically connected to a signal detection terminal 16 disposed on the upper lid 14. The lower end portion of the by-pass casing 2a is communicated with valve casing 11a, an armature 11d having a sealing material 11c at the lower end portion thereof is internally provided in a liftable manner in the valve casing 11a through a spring 17, and an orifice 11e communicating with the outlet port 8c of the housing 8 is defined at the lower position of the armature 11d of the valve casing 11a. Furthermore, an electromagnetic coil 9 is rigidly mounted on the outer circumferential portion of the reduced diameter portion 8a of the housing 8 which is positioned on the outside of the armature 11d, and the armature 11d is arranged to be lifted and lowered by means of a magnetic field generated by the electromagnetic coil 9.
Moreover, the signal detection terminal 16 connected to the mass flow sensor 10 is connected with a flow detection amplifier A, this flow detection amplifier A is connected to a servo amplifier D through a linearization circuit B and an output amplifier C, and the servo amplifier D is connected to the electromagnetic coil 9 through a valve driver E. The servo amplifier D is further connected to a flow rate setting device F, and a flow rate setting signal is compared with a signal which is input from the output amplifier C.
In the above construction, when a prescribed gas is supplied from the inlet side of the fluid conduit 1, the gas passes through the filter 5 in the flow through passage 6b of the base block 6 and flows into the inlet port 8b defined in the housing 8 of the mass flow controller unit 7, and thereafter the gas flows into the by-pass casing 2a through the gas passageway 13, the communicating hole 11b in the valve casing 11a and the gas passageway 15. A part of the gas which flowed into the by-pass casing 2a passes through the mass flow sensor 10, while the remainder passes through the by-pass elements to flow into the valve casing 11a. In this case, the gas supplied is detected by the mass flow sensor 10 as a temperature difference signal and is input to the servo amplifier D as a flow rate output signal via the flow rate detection amplifier A, the linearization circuit B and the output amplifier C. In the servo amplifier D, the flow rate output signal is compared with a flow rate setting signal which has been input from the flow rate setting device F, whereby a corrected signal is delivered to the valve driver E so that suitable electric current is applied to the electromagnetic coil 9. On the other hand, the gas which passes through the by-pass casing 2a and flows into the valve casing 11a passes through the outer circumferential part of the armature 11d and flows into the orifice 11e. In this case, the armature 11d is suitably adjusted to be lifted or lowered by means of the magnetic field of the electromagnetic coil 9 as a function of the corrected signal received from the servo amplifier D. As a result, the flow rate of the gas flowing out to the outlet side of the fluid conduit 1 from the orifice 11e and the outlet port 8c of the housing 8 is controlled to be a value which is very close to a flow rate value which has been preset.
However, since the mass flow control unit 7 is vertically disposed with respect to the fluid conduit 1 in such conventional mass flow controller, the dead volume becomes high. In addition. such conventional mass flow controller is arranged such that the gas which passes through the fluid conduit 1 flows into the inlet port 8b, then the gas rises to pass through the gas passageways 13 and 15 and flows into the by-pass casing 2a, and thereafter the gas passes through the valve casing 11a and flows to the outlet side of the fluid conduit 1 from the outlet port 8c. Accordingly, there arises easily a so-called gas accumulation wherein flowing gas accumulates in the vicinity of the inlet port 8b or the outlet port 8c under the above-mentioned conditions. For this reason, such gas accumulation has been an obstacle in the case of internal gas exchange.
Furthermore, there is such a problem that resistance to gas flow becomes high, because the gas passageways are of complicated construction inside the flow rate control unit 7. Further, such construction of the gas passageways also is disadvantageous from the viewpoint of maintenance.